J. Phys. Chem. B 1999, 103, 4663-4665
4663
Observation of One Process in a Phase Transfer Catalytic Reaction at a Liquid/Liquid
Interface by Using the Quasi-Elastic Laser Scattering Method
Yoshiko Uchiyama, Isao Tsuyumoto, Takehiko Kitamori, and Tsuguo Sawada*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo,
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
ReceiVed: December 16, 1998; In Final Form: March 15, 1999
The interfacial behavior of a phase transfer catalyst, tetrabutylammonium bromide (TBAB), was investigated
by the quasi-elastic laser scattering (QELS) method. TBAB forms an ion pair (TBA+C6H5O-) with sodium
phenoxide (C6H5ONa) in a water phase. This ion pair moves from the water phase to the organic phase
where it reacts with diphenylphosphoryl chloride (DPPC). This one process in the phase transfer catalytic
reaction at a water/nitrobenzene (W/NB) interface was investigated by the QELS method. When the TBAB
concentration was above 50 mM, the ratio of TBAB concentration to C6H5ONa in equilibrium at the W/NB
interface was unity. On the other hand, when the TBAB concentration was below 50 mM, the ratio was
below unity. These results suggested that the reaction place between TBAB and C6H5ONa changed with
TBAB concentration; the reaction place was in the water phase above 50 mM, whereas it was at the interface
below 50 mM.
Introduction
The liquid/liquid interface holds important roles in various
processes such as phase transfer catalysis, solvent extraction,
and ion-selective electrode operation. Among them, phase
transfer catalysis has attracted much attention because a reaction
is promoted by transfer across the liquid/liquid interface to get
high product yield and high selectivity.1-5 Tetrabutylammonium
bromide (TBAB), a well-known surfactant, is a phase transfer
catalyst, which circulates between the two phases across the
interface. Therefore, the investigation of the behavior of TBAB
at the liquid/liquid interface is essential to understand the overall
catalytic reaction.
Many studies have been reported on the behavior of a phase
transfer catalyst based on conventional batch methods using
NMR, UV-vis spectrophotometry, and gas chromatography,
etc., by taking out part of the solution from the bulk phase.6-13
Thus, the behavior of the phase transfer catalyst has been mainly
analyzed by information from the bulk phase. However, for
better understanding, direct measurement of the interface is
essential. There are a number of studies that measure interfacial
tension to provide useful information on the interface. In these
studies, the interfacial tension was measured by the Wilhelmy
method, drop method, etc., which perturbates the interface and
causes a certain amount of change in the interfacial tension.14-17
These techniques cannot be adapted to precise measurements
of interfacial behavior of the phase transfer catalyst. Thus, the
behavior of a phase transfer catalyst at the liquid/liquid interface
has not been directly measured so far, and the reaction rate and
field of its elementary processes such as adsorption, desorption,
and reaction between the two chemical species still remain
unclarified. In situ measurements of liquid/liquid interfaces
should provide great insight into interfacial molecular behavior
of phase transfer catalyst.
We have developed a quasi-elastic laser scattering (QELS)
method and have reported on the dynamics of mass transfer of
surfactants at a water/nitrobenzene (W/NB) interface.18-22 We
demonstrated that the QELS method is a useful spectroscopic
Figure 1. Principle of the quasi-elastic laser scattering method.
technique for nonperturbative in situ measurements of liquid/
liquid interface.
In this study, we investigated a phase transfer catalytic
reaction system with TBAB, whose reaction scheme is wellknown and the operating conditions are mild. We focused on
the behavior of TBAB at the W/NB interface in one simple
process of phase transfer catalytic reaction.23,24 Our aims are to
obtain direct information about the interface from in situ
measurements and to clarify the reaction place around the
interface from the interfacial molecular behavior. We present
here the results for in situ measurements of interfacial adsorption
behavior, which suggest the reaction place changes with the
concentration.
Experimental Section
Principle. The principle of the QELS method has already
been described previously.18-22 In brief, an incident laser beam
normal to the interface is scattered quasi-elastically with a
Doppler shift by the capillary wave (Figure 1). The scattered
beam is optically mixed with a local beam produced by a
diffraction grating to generate an optical beat in the mixed light.
10.1021/jp984756k CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/05/1999
4664 J. Phys. Chem. B, Vol. 103, No. 22, 1999
Uchiyama et al.
Figure 2. Schematic diagram of the experimental setup: T, glass tube;
W, water phase; NB, nitrobenzene; PD, photodiode; AMP, preamplifier;
FFT, FFT analyzer.
Figure 4. Capillary wave frequency dependence on TBAB concentration.
Figure 3. Scheme of the reaction between C6H5ONa and DPPC in
the W/NB system. The part surrounded by the dashed line is the process
investigated in this study.
The obtained beat frequency is the same as the Doppler shift,
i.e., the capillary wave frequency.
The capillary wave frequency f is approximately expressed
by Lamb’s equation,25
f)
(
)
1
γ
2π Fw + Fo
1/2
k3/2
(1)
where Fw is the density of the water phase, Fo is the density of
the organic phase, and k is the wavenumber of the capillary
wave. Because the interfacial number density Γ of a surfactant
monolayer is approximately inversely proportional to the
interfacial tension γ, the relative change of the number density
Γ can be estimated from the capillary wave frequency.
Apparatus. A schematic diagram of the experimental setup
is shown in Figure 2. The beam from a YAG laser (CrystaLaser,
model GCL-025S, 532 nm, 20 mW) passes through a transmitting diffraction grating in front of the cell. The cell is made of
quartz glass and has an optically flat bottom. After passing
through the sample, one of the diffracted beams, which is mixed
with the scattered light, is selected by the aperture in front of
the photodiode (Hamamatsu Photonics S1290). Signals from
the photodiode are Fourier transformed and saved by a digital
spectrum analyzer (Sony Tektronix Co., model 3056). In the
present study, the wavelength of the observed capillary wave
was 6.6 × 10-3 cm.
Sample Preparation. The scheme of the reaction between
sodium phenoxide (C6H5ONa) and diphenylphosphoryl chloride
(DPPC) in a W/NB system by TBAB is shown in Figure 3. At
the beginning of the reaction, TBAB reacts with C6H5ONa to
form TBA+C6H5O-. This ion pair is transported into the organic
phase where it reacts with DPPC to produce triphenyl phosphate
((C6H5O)3PO). During this reaction, TBA+Cl- is also formed
and transported into the water phase to react again with C6H5-
Figure 5. Capillary wave frequency dependence on the concentrations
of TBAB and C6H5ONa.
ONa. TBAB activates the production of triphenyl phosphate
by circulating between the two phases.
In this study, we focused on one transfer process in which
the formed ion pair is transferred from the water phase to the
nitrobenzene phase. Thus, we investigated the adsorption
behavior of the ion pair at various concentrations of TBAB
(0-80 mM) and C6H5ONa (0-100 mM). A liquid/liquid
interface was prepared by adding 10 mL of mixed aqueous
solution of TBAB (Kanto Chemical Co.), C6H5ONa (Aldrich),
and NaOH (Kanto Chemical Co.) to 10 mL of nitrobenzene in
a quartz cell. Ultrapure water (from Millipore Milli-Q system)
was used for all sample preparations. NaOH was added to adjust
the ionic strength to 0.2 and to prevent the production of phenol
or benzoic acid. All chemicals were reagent grade and used
without further purification.
Results and Discussion
Capillary wave frequencies at various concentrations of
TBAB alone are shown in Figure 4. The frequency decreased
significantly with increasing TBAB concentration and became
constant at about 100 mM. This indicated that interfacial tension
was decreased by the interfacial adsorption of TBAB, and the
interfacial adsorption became saturated at about 100 mM.
Capillary wave frequencies at various concentrations of TBAB
and C6H5ONa are shown in Figure 5. When C6H5ONa alone
was in the water phase, the frequency was independent of the
C6H5ONa concentration. On the other hand, when both C6H5ONa and TBAB were present, the frequency decreased gradually
and only slightly with increasing C6H5ONa concentration and
then became constant at a certain C6H5ONa concentration. These
Phase Transfer Catalytic Reaction
J. Phys. Chem. B, Vol. 103, No. 22, 1999 4665
Figure 6. Relationship between the TBAB and C6H5ONa concentrations in equilibrium.
facts indicated that C6H5ONa molecules did not adsorb onto
the interface alone, and the adsorption of C6H5ONa molecules
was promoted in the presence of TBAB. Therefore, the decrease
of frequency indicated that TBAB formed an ion pair with C6H5ONa, such as TBA+C6H5O-, and these ion pairs adsorbed at
the interface. The constant frequency above a certain concentration indicated that the number of interfacial adsorptions of ion
pairs that occurred became constant. This suggested that the
reaction between C6H5ONa and TBAB was in equilibrium, and
the number of ion pairs that occurred became constant. This
stable formation brought about the saturated interface. The
formation of ion pairs corresponded to findings in other
reports.23,24 However, saturation of the interfacial adsorption
above a certain concentration of C6H5ONa has not been reported
so far, and this provided significant information on the reaction
between TBAB and C6H5ONa.
Furthermore, interestingly, the equilibrium concentration of
C6H5ONa above which the adsorption was saturated depended
on the TBAB concentration. We used these equilibrium
concentrations to analyze the reaction between the two reactants,
assuming that at these concentrations the reaction proceeds
without residue and deficiency. The relationship between this
C6H5ONa concentration and the TBAB concentration is shown
in Figure 6. We noted that the ratio of the TBAB concentration
to the C6H5ONa concentration deviated from the line for 1:1
below 50 mM. When the TBAB concentration was above 50
mM, the ratio of the TBAB concentration to the C6H5ONa
concentration at the W/NB interface was unity. On the other
hand, when the TBAB concentration was below 50 mM, the
ratio of the C6H5ONa concentration to the TBAB concentration
was more than unity.
In general, the formation of an ion pair TBA+C6H5O- occurs
by the reaction between one TBAB molecule and one C6H5ONa molecule in the water phase. This can be empirically
deduced from studies on the bulk phase concentrations.6-13
Thus, the ratio of the C6H5ONa concentration to the TBAB
concentration should be unity if we assume the reaction occurs
in the water phase. The behavior above 50 mM can be simply
explained by the reaction in the water phase between one TBAB
molecule and one C6H5ONa molecule because the ratio is unity.
However, the behavior below 50 mM cannot be explained by
the simple water phase reaction. This result suggests that the
reaction place is not in the water phase. It is generally said that
the concentration or activity at the interface is different from
that in the water phase. Thus, if we assume the ratio of the two
concentrations at the interface is unity and the reaction between
the TBAB molecules and the C6H5ONa molecules occurs at
the interface, the deviation from the line of 1:1 can be explained.
As shown in Figure 7, our results suggested that below 50
mM the reaction of the formation of the ion pair occurred at
Figure 7. Model of the reaction between TBAB and C6H5ONa.
the interface while above 50 mM the reaction occurred in the
water phase.
Our results reflected interfacial specificity for a chemical
reaction and mass transfer. We successfully observed the
molecular behavior reflecting the interfacial specificity focusing
on one process of the phase transfer catalyst system using the
QELS method. In future studies the advantages of the QELS
method can be used to provide details on interfacial specificity
for chemical processes by nonperturbative measurements.
Acknowledgment. The authors are grateful to Prof. Noritaka
Mizuno of the University of Tokyo, School of Engineering, for
valuable discussions.
References and Notes
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